[Phys-L] Lenz's law

[pschoch <pschoch@ptd.net>]

So, here's where I get myself into trouble.   I encourage my students to 
understand why things happen, what causes them, and not just memorize the 
formulas.  And now we've gotten to Induction -- Lenz's law. 
  
The textbook shows them that the induced EMF opposes the change causing it, 
thus the negative sign -- experimentally.  Now I've had a student ask me what 
the underlying physical principle is that results in the negative sign.   
Honestly, I don't think I understand this well enough to give an explanation. 
  
Any help would be appreciated, 
Peter Schoch